There are a few reasons that I like Milotic that I will explain. Namely, it isn't really outclassed by Water-types imo and its usually one that I go for a lot, even though most people don't like it.
Milotic is one of only three viable bulky waters in the tier with an instant recovery move, with the others being, as you mentioned, Slowking and Slowbro. I hate getting worn down over the battle, especially considering that Fire-types are all over the place out there. If you get worn down and you encounter a Life Orb Darmanitan or Chandelure, you're screwed. However, that's where Recover comes in handy.
The reason why I like using Milotic over Slowking and Slowbro comes down to a few reasons. The first of these is Roar. Although Slowking gets Dragon Tail, it's an extremely weak move and is pretty much a Roar that can miss that occasionally fails against substitutes, and then Slowbro has absolutely nothing at all to phaze its foes. Milotic's lack of a Psychic-typing is both a blessing and a curse – on one hand, it doesn't need to worry about Chandelure's Shadow Ball (it can switch into the move, heal up as it uses Energy Ball, and then pretty good from there), U-turn, Heracross's Megahorn, and Dark moves. Dark moves are everywhere in UU, and the ability to not get destroyed by Weavile, Krookodile to an extent, Bisharp, Zoroark without Grass Knot, and Honchkrow is a nice feature. Milotic also has a decent speed, meaning it can attempt to Scald or Toxic Snorlax before it can do massive damage to the team. Usually, I run Slowbro when I need a bulky water that resists Fighting (which is often), but when I need just a general good bulky water and don't need the additional typings or Stealth Rock, Milotic is usually a good choice in my opinion, and even if it does stop momentum a little bit with its recover spamming, Toxic wears down the opponent, and same with a potential burn. It's also insanely bulky - it has 95/79/125 bulk, and although stats don't make the Pokemon by any means, that's really bulky, especially on the special side. It also makes a good Scald switch in due to Marvel Scale - if that activates, have fun getting through it.
Otherwise it's not great, but Milotic isn't necessarily outclassed in the metagame imo. I've never thought Milotic was bad and its never hurt my team really, and I feel the same about Gligar, Hitmontop, and to an extent Registeel.